It’s been an eventful week for Mars-related activities. After suffering a software reset on November 7th, as reported on in my last MSL update, the rover Curiosity experienced a second problem on November 17th.
This was caused by an unexpected voltage change being detected in the vehicle, described as a “soft” short, meaning that electricity is unexpectedly passing through something that is partially conductive, and in difference to a “hard” short, such as one electrical wire contacting another.
The short was first noted as a voltage difference between the chassis and the 32-volt power bus that distributes electricity to systems throughout the rover. Data indicating the change were received on Sunday November 17th, Curiosity’s 456th Martian day. Prior to the short occurring, the voltage level had been a consistent 11 volts; however, the data received indicated it has dropped to 4 volts.
While there was no immediate danger, as the rover’s electrical system is designed with the flexibility to work properly across a range of voltages – a design feature called “floating bus” – the decision was taken to suspend science operations while matters were investigated.
This was actually the second soft short Curiosity has experienced. The first occurred on the very day it arrived on Mars – August 5th/6th 2012 – when the bus-to-chassis voltage dropped from about 16 volts to about 11 volts. This was thought to be related to explosive-release devices used for deployments shortly before and after the landing, but did it not and has not interfered with the rover’s operational capacity or capabilities.
Although the voltage change did not pose any immediate threat, and the vehicle did not enter a safe mode status, nor was it related to the earlier software reset, such soft shorts can reduce the robustness of the rover’s electrical systems for tolerating other shorts in the future. Further, they can indicate a possible problem in whichever component is the site of the short. Hence the decision to suspend science operations and take time to check some of the possible root causes for the voltage change.
Subsequent analysis revealed that the voltage drop occurred intermittently three times in the hours before it became persistent. Some six days were spent in root cause analysis using data returned by the rover, which saw a number of potential causes suggested by mission engineers eliminated. This resulted in the most likely cause being identified as an internal short in Curiosity’s power source, the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG or RTG for short).
Due to resiliency in the RTG design, this type of short does not affect operation of the power source or the rover. In fact, similar generators on other spacecraft, including NASA’s Cassini at Saturn, have experienced shorts with no loss of capability, and testing of an Earth-based RTG over many years found no loss of capability despite the presence of these types of internal shorts.
As a result of these findings, the rover was commanded to re-start science activities on November 23rd, and data returned from Curiosity’s onboard monitoring systems indicated that voltage levels had successfully returned to levels prior to that of the November 17th incident, a sign which is again indicative of an internal short within the RTG systems.
The resumption of science activities was marked by the rover delivering a further sample of rock cutting gathered some 6 months ago from an outcrop dubbed “Cumberland” in the “Yellowknife Bay” area of Gale Crater. A number of samples from the outcrop have already been analysed by the Sample Analysis at Mars (SAM) suite of instruments, which has the flexibility to be able to carry out such analyses a number of different ways, allowing significantly more data to be gathered on samples of the same rock gathered and stored by the rover.
MAVEN en route to Mars
NASA’s next orbital mission to Mars was successfully launched on November 18th, 2013. The Mars Atmosphere and Volatile Evolution (MAVEN) vehicle is designed to study the Martian atmosphere with the aim of determining how the Martian atmosphere and water, presumed to have once been substantial, were lost over time.
MAVEN was launched from Canaveral Air Force Station (the complex of launch facilities operated by the US Air Force alongside NASA’s own Kennedy Space Centre facilities, and long used for unmanned launches). The launch vehicle comprised an Atlas V vehicle operated by the Lockheed Martin-Boeing joint venture United Launch Alliance.
The Atlas V is a two-stage vehicle using a Russian RD-180 engine in the first stage and a Centaur upper stage, unique in being the world’s first high-energy upper stage using liquid hydrogen and liquid oxygen as its fuel mix. Centaur has a long history in the US space programme, starting in 1965 with its first successful launch. Since then, it has been responsible for boosting many satellites to geostationary orbit above the Earth and for boosting many famous deep-space missions on their way, including the Viking 1 and Viking 2 missions to Mars in the 1970s, and the Voyager space craft.
The MAVEN launch was a direct-to-Mars trajectory mission which saw the Atlas V place the probe into an elliptical path around the Earth using both the first and second (Centaur) stages in a 14-minute flight. This was followed by a “coast” phase of some 27 minutes as the vehicle, still attached to the Centaur upper stage, flew part-way around the Earth coasting upwards and around towards Mars. The Centaur motor was then re-lit, boosting MAVEN onto what is called a Trans-Mars Injection (TMI) orbit prior to it separating from the spacecraft, allowing it to deploy its solar panels. Some three hours after launch, MAVEN was already some 22,400 kilometres (14,000 miles) into its journey, and it should arrive in orbit around Mars in September 2014.
MAVEN’s mission is intended to further probe the mysteries of how Mars lost what must have once been a plentiful and dense atmosphere – dense enough for liquid water to have once existed on the planet’s surface. By measuring how quickly the atmosphere escapes into space and the relevant processes, scientists hope to understand how the planet’s atmosphere evolved. As such, MAVEN has four primary scientific objectives:
- Determine the role that loss of volatiles to space from the Martian atmosphere has played through time.
- Determine the current state of the upper atmosphere, ionosphere, and interactions with the solar wind.
- Determine the current rates of escape of neutral gases and ions to space and the processes controlling them.
- Determine the ratios of stable isotopes in the Martian atmosphere.
However, the vehicle will not be operating entirely on its own. India’s Mangalyaan vehicle set-off for Mars ahead of MAVEN, and will also be studying the Martian atmosphere as a part of a broad set of goals for the mission after it arrives there also in September 2014. In the meantime, Curiosity is using the multi-purpose SAM instrument to sample and analyse the Martian atmosphere at ground-level, further adding to the amount of data which can be obtained.
All images and videos courtesy of NASA / JPL